Is the salary received from "pre-employment contracts" considered halal or haram, knowing that employers ask employees not to come to work due to a lack of vacancies, and that the state has created these contracts to absorb unemployment and the anger of youth?

It is permissible for you to benefit from the salaries you receive if the contracting party is the one who asked you to stay in your homes, because you have enabled the hirer to benefit from your services. The wage becomes due upon the employee's submission of himself during the period, even if he does not perform work. However, whenever the employer asks you to be present, you must comply, otherwise the salary becomes unlawful for you.